The present invention relates to a method and equipment for connecting optical communication systems using different wavelength bandwidths, for example connecting a system using 1.3 xcexcm-band optical wavelength and a system using 1.5 xcexcm-band optical wavelength, thereby to transmit signals therebetween corresponding to each signal type consisting of either RZ (return-to-zero) code or NRZ (non-return-to-zero) code.
An optical wavelength converter is particularly beneficial to a wavelength division multiplexing (WDM) optical communication system, in which data are transmitted in a plurality of channels through a common optical fiber using multi-wavelength signal light.
As the amount of transmission data through a backbone optical communication system abruptly increases, the larger capacity is required in an optical communication system. As a method therefor, a wavelength division multiplexing (WDM) optical communication system has been started in use.
A WDM optical communication system is developed mainly using optical wavelengths in 1.5 xcexcm band. On the other hand, conventional optical terminal station systems employ 1.3 xcexcm band. Therefore, optical wavelength conversion is necessary for connecting these systems.
In FIG. 1, there is shown a schematic configuration diagram of a WDM optical communication system. In the existing optical terminal station system employing 1.3 xcexcm band, optical signals having a plurality of optical wavelengths in 1.3 xcexcm band are generated. A plurality of 1.3 xcexcm band optical signals are respectively converted into optical signals having different wavelengths of 1.5 xcexcm band in an optical wavelength converter 102.
Then, optical signals having different wavelengths of 1.5 xcexcm band are wavelength-multiplexed in a multiplexing circuit 110 of a 1.5 xcexcm WDM transmission system 103 to transmit through an optical fiber transmission line 111.
At the receiving side of 1.5 xcexcm WDM transmission system 103, wavelength-multiplexed optical signals transmitted through optical fiber transmission line 111 are received to be divided into each optical signal having respective wavelength by a demultiplexing equipment 112.
The wavelength-divided optical signals of 1.5 xcexcm band are converted into optical signals of 1.3 xcexcm band by an optical wavelength converter 104 to transmit to 1.3 xcexcm WDM transmission system 105.
Here, the transmitted optical signals are formed of either NRZ (non-return-to-zero) code or RZ (return-to-zero) code.
In FIG. 2, there are shown conventional configurations of optical wavelength converters 102 and 104 illustrated in FIG. 1. In particular, FIGS. 2A and 2B respectively show optical wavelength converters 102 and 104 in case the optical signal is formed of NRZ code. FIGS. 2C and 2D show optical wavelength converters 102 and 104 in case the optical signal is formed of RZ code.
In FIGS. 2A and 2B, there are shown optical wavelength converters 102 and 104 having identical configuration, provided with an optical-to-electrical converter 203 and an electrical-to-optical converter 204 coupled by a capacitor C. Electric outputs of a non-inverted signal (DATA) and an inverted signal (NDATA) having been converted into an electric signal by optical-to-electrical converter 203 are led to electrical-to-optical converter 204.
In electrical-to-optical converter 204, the signal is converted into a corresponding optical signal having wavelength of either 1.5 xcexcm band or 1.3 xcexcm band.
Also, in FIGS. 2C and 2D, the configurations of optical wavelength converters 102 and 104 are identical, each provided with an optical-to-electrical converter 203 and an electrical-to-optical converter 204 coupled by a capacitor C. Electric outputs of a non-inverted signal (DATA) and an inverted signal (NDATA) having been converted by optical-to-electrical converter 203 are led to electrical-to-optical converter 204.
Moreover, in the configurations shown in FIGS. 2C and 2D, a bias voltage is applied to the output of the capacitor C from a bias adjustment circuit 205.
The reason for requiring this bias adjustment circuit 205 in case of RZ code is explained later.
A configuration example of optical-to-electrical converter 203 used in the aforementioned optical wavelength converters 102 and 104 is shown in FIG. 3.
In FIG. 3, optical signals transmitted through an optical fiber 201 or 202 are received by a photo diode PD to convert into an electric signal of which magnitude correspond to the magnitude of an optical signal. The electric signal is amplified by a pre-amplifier 206, and is output to a signal having a limited amplitude adjusted by a waveform shaping circuit 207.
In FIG. 4, there are shown waveforms in various parts of optical-to-electrical converter 203 shown in FIG. 3 in the cases of RZ code and NRZ code. Waveforms in case of RZ code are shown on the left column, and waveforms in case of NRZ code are shown on the right column.
There are respectively shown outputs [a] and [b] from pre-amplifier 206 in FIG. 4A, inputs [axe2x80x2] and [bxe2x80x2] to waveform shaping circuit 207 in FIG. 4B, and output waveform [axe2x80x3] and [bxe2x80x3] from electrical-to-optical converter 203 in FIG. 4C.
Now the case of RZ code is described hereafter. As shown in FIG. 4A, a mean value of non-inverted output DATA ([a]) output from pre-amplifier 206 locates lower than the center of amplitude [O], and a mean value of inverted output NDATA ([b]) locates higher than the center of amplitude [O]. This is because the period of L level is longer than the period of H level in consequence of the nature of RZ code.
Accordingly, a waveform of signal obtained through a capacitor C to be input to waveform shaping circuit 207 is changed with the center of amplitude shifted for a bias voltage. In an example shown in FIG. 4B, the waveform is shifted for approximately one-fourth of the amplitude.
Input signals [axe2x80x2] and [bxe2x80x2] are shaped by waveform shaping circuit 207 into waveforms each having a constant amplitude. However, because the phase is shifted, the duty remains deteriorated, as shown in FIG. 4C.
Now the case of NRZ code is described, referring to the diagrams shown on the right side of FIG. 4. In this case, because of the nature of NRZ code, the period of H level and the period of L level is substantially identical, producing no bias voltage. Therefore, the duty is not deteriorated in the output of waveform shaping circuit 207.
In FIG. 5, there is a diagram for illustrating the reason for providing a bias adjustment circuit 205. Bias adjustment circuit 205 is provided to solve the problem of duty deterioration produced in case of RZ code as shown in FIG. 4, caused by the bias voltage shifted in optical-to-electrical converter 203. Bias adjustment circuit 205 is provided in optical wavelength converter 104 as shown in FIGS. 2C and 2D.
In FIG. 5A, there are illustrated waveforms [axe2x80x3] and [bxe2x80x3] output from optical-to-electrical converter 203 corresponding to FIG. 4C. As understood from the figure, mean values of non-inverted output signal DATA and inverted output signal NDATA are shifted from the center of amplitude caused by RZ code.
Therefore, in FIG. 2, an input signal to electrical-to-optical converter 204 through the capacitor C has a deviated direct-current component resulting in a deteriorated duty, as shown in FIG. 5B. As a measure therefor, bias adjustment circuit 205 is provided on the input side of electrical-to-optical converter 204, as shown in FIGS. 2C and 2D.
Accordingly, the duty of a waveform shown in FIG. 5B is improved by adjusting the bias voltage, as shown in FIG. 5C.
In the conventional transmission line, a wide range of transmission rate varying from low speed to high speed is used, and also both RZ code and NRZ code are used as transmission line code. Therefore, as mentioned above, it is necessary to prepare an optical wavelength converter corresponding to the transmission line code.
In an optical wavelength converter for connecting a system employing optical wavelengths of 1.3 xcexcm band and a system employing optical wavelengths of 1.5 xcexcm band, a conventional optical wavelength converter has the following problems to realize bit-free transmission using RZ or NRZ code, as having been explained in FIGS. 2 to 5.
First, each different optical wavelength converters are required corresponding to RZ or NRZ code. An electrical-to-optical converter and an optical-to-electrical converter employed in an optical wavelength converter have different interface levels of electric signals depending on manufactures.
For example, there are an optical-to-electrical converter having ECL (emitter coupled logic) level, and an electrical-to-optical converter having either ECL level or CML (current mode logic) level, and so forth. Also, there are different values of standard direct current (DC) potential such as xe2x88x921.3 V in ECL level and xe2x88x920.5 V in CML level.
Is such a case as having different standard DC potential, normally using capacitive coupling, an input bias voltage of an electrical-to-optical converter has to be changed corresponding to either RZ or NRZ code. This requires optical wavelength converters to be provided individually as shown in FIG. 2.
Secondly, there is a problem that input amplitude margin to the electrical-to-optical converter may be deteriorated. Generally, capacitive coupling is used in the optical-to-electrical converter as shown in FIG. 3. Capacitive coupling is also used in the electrical-to-optical converter.
For this reason, in case of RZ code, the duty shifts in the output waveform of the optical-to-electrical converter, as shown in FIG. 4. Furthermore, after the C coupling in the optical wavelength converter, the duty is shifted as shown in FIG. 5B. To cancel this, the duty is corrected in the electrical-to-optical converter by adjusting the bias voltage as shown in FIG. 5C. This produces decreased input amplitude, resulting in failing to satisfy the input condition against the specification of electrical-to-optical converter 204.
Thirdly, the shift in an optical input waveform produces deterioration in input amplitude margin of electrical-to-optical converter 204, and affects the duty of an optical output waveform.
The correction is carried out so that the duty ratio of an output waveform of electrical-to-optical converter 204 becomes 50% when the duty ratio of an input waveform of optical-to-electrical converter 203 is equal to 50%.
Therefore, when the duty of the input optical waveform is shifted, the duty of the output optical waveform is also shifted. However, because the input bias voltage of electrical-to-optical converter 204 is of fixed value, the shift in the input of electrical-to-optical converter 204 affects the output duty. Moreover, there may be a case that the input amplitude of electrical-to-optical converter 204 becomes small, resulting in failing to satisfy input specification condition.
It is therefore an object of the present invention to provide an optical wavelength converter whereby the above-mentioned problems can be solved.
The basic configuration of an optical wavelength converter according to the present invention is described hereafter. An optical wavelength converter for converting an optical signal in a first optical wavelength band into an optical signal in a second optical wavelength band includes: an optical-to-electrical converter for converting an input optical signal into an electric signal; an electrical-to-optical converter for converting an electric signal into an optical signal; and a duty control circuit connected between the optical-to-electrical converter and the electrical-to-optical converter for controlling the duty of the electric signal converted by the optical-to-electrical converter corresponding to either RZ code or NRZ code.
Preferably, the optical signal in the first optical wavelength band and the optical signal in the different second optical wavelength band respectively have wavelengths in 1.3 xcexcm band and wavelengths in 1.5 xcexcm band.
Preferably, the duty control circuit includes a mean value detection circuit for detecting respective mean values of non-inverted data of the electric signal and inverted data thereof; and a level converter controlled so that the mean values of the non-inverted data and the inverted data detected by the mean value detection circuit coincide with each other.
Furthermore, preferably, the level converter provides a differential pair circuit having both the non-inverted data and the inverted data of the electric signal as inputs thereof, to be connected to the electrical-to-optical converter either through an interface having resistors respectively inserted between both output drains (or collectors) of the differential pair and the ground, or through an interface having a common resistor inserted between both output drains (or collectors) and the ground.
Still further, preferably, a limiter circuit is provided for limiting a control signal level to control the level converter.
An optical wavelength division multiplexing system according to the present invention includes: multiplexing equipment for wave-multiplexing an optical signal in a second wavelength band converted from an optical signal in a first wavelength band by the aforementioned optical wavelength converter; an optical fiber for transmitting the optical signal wave-multiplexed by the multiplexing equipment; demultiplexing equipment for wave-demultiplexing the optical multi-wavelength signal transmitted through the optical fiber; and the aforementioned optical wavelength converter for converting each wavelength of optical signal having the second wavelength band demultiplexed by the demultiplexing equipment into optical signal having the first wavelength band.
Further scopes and features of the present invention will become more apparent by the following description of the embodiments with the accompanied drawings.